Proceedings of the Royal Society of Edinburgh. 189 



their distinctness a maximum when the sun sinks beneath the hori- 

 zon. The study of them consequently becomes exceedingly difficult 

 in a climate where the sun, even in a serene day, almost always sets 

 in clouds ; but the author has been able to execute a tolerably accu- 

 rate delineation of the atmospheric spectrum. 



Most of the lines thus widened by the atmosphere, are faint lines 

 previously existing in the spectrum. 



The author's observations, whilst they indicate the remarkable fact, 

 that the same absorptive elements which exist in nitrous acid gas 

 exist also in the atmospheres of the sun and of the earth, lead us to 

 anticipate very interesting results from the examination of the spec- 

 tra of the planets. Fraunhofer had observed in the spectra of Venus 

 and Mars some of the principal lines of the solar spectrum. This, 

 indeed, is a necessary consequence of their being illuminated by the 

 sunj for no change which the light of that luminary can undergo is 

 capable of replacing the rays which it has lost. But while we must 

 find in the spectra of tlie planets and their satellites all the defective 

 lines in the solar spectrum, we may confidently look for others aris- 

 ing from the double transit of the sun^s light through the atmospheres 

 Avhich surround them. 



2. Notice relative to the Pigmentum Nigrum of the Eye. 

 By Thomas Wharton Jones, Esq. 



The objects proposed by the author of this paper are, — 



1. To correct certain opinions prevalent with regard to the mem- 

 brane of Jacob, this having been frequently confounded with ano- 

 tlier, the structure of which forms the immediate subject of the 

 paper. 



2. To shew that the Pigmentum Nigrum is not a mere mucus or 

 varnish exhaled by the surfaces on which it is found, but is deposited 

 in a membrane distinct from the choroid, which possesses a peculiar 

 structure hitherto unknown. This membrane, being the seat of the 

 pigment, but not the pigment itself, which may or may not be pre- 

 sent, the author proposes to call the Membrane of the Pigment. 



If a portion of this membrane be examined by the aid of the 

 microscope, it is seen to consist of very minute hexagonal plates, in 

 which are deposited numerous black particles, which are to be consi- 

 dered as properly constituting the pigment, but not essential to the 

 hexagonal plates composing the membrane, because these may, and 

 do, exist without the black particles. 



In the eye of the Albino Rabbit, the author found, as he had a 

 priori expected, the membrane of the pigment to exist. The plates 



